Mechanical ventilation causes rapid diaphragm muscle atrophy, detectable within 18 hours of ventilator support, which is a major contributor to weaning failure and prolonged ICU stay.

This randomized controlled trial tests whether a physiotherapist-guided inspiratory endurance training program - achieved by progressively reducing pressure support (PS) levels on the ventilator - can preserve or improve inspiratory muscle strength and diaphragm structure in critically ill adults on mechanical ventilation, compared to standard ICU care.

Participants will be randomly assigned to one of two groups. The intervention group will receive twice-daily training sessions (weekdays only) in which the physiotherapist gradually reduces PS by up to 50% of the baseline level, causing the patient to breathe with greater muscular effort - equivalent to progressive endurance exercise for the diaphragm. The control group will receive standard weaning care as directed by the attending intensivist.

The primary outcome is maximal inspiratory pressure (PImax) at day 7. Secondary outcomes include diaphragm thickening fraction measured by ultrasound, time to successful extubation, weaning success rate, and mortality.

All measurements are performed by the ICU physician (blinded to group allocation) using airway occlusion maneuvers recorded on the ventilator and diaphragm ultrasound, following a standardized protocol.

Inclusion Criteria:

1. Age ≥ 18 years
2. Admitted to ICU with invasive mechanical ventilation for more than 48 hours at the time of randomization
3. Expected to remain intubated for at least 24 additional hours, at the discretion of the attending intensivist
4. Active pressure support ventilation (PSV) mode with PS level ≥ 5 cmH2O
5. Richmond Agitation-Sedation Scale (RASS) score between -1 and +1
6. Arterial pH ≥ 7.35 and serum lactate ≤ 2.0 mmol/L
7. Peripheral oxygen saturation (SpO2) ≥ 90% with current FiO2
8. Informed consent obtained from patient or legal representative

Exclusion Criteria:

1. Neurological condition affecting central respiratory control (high cervical spinal cord injury, ALS, myasthenia gravis, active Guillain-Barré syndrome)
2. Established tracheostomy at the time of inclusion
3. Restriction on head-of-bed elevation to ≥ 30 degrees
4. Decision to limit therapeutic effort (LET) or life expectancy < 48 hours
5. Active hemodynamic instability (requirement for ≥ 2 vasopressors or vasopressor adjustment within the last 6 hours)
6. Multiple rib fractures or undrained pneumothorax
7. Simultaneous participation in another interventional clinical trial
8. Previous inclusion in this study